Thermoelectric oxides for waste heat recovery
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Among various solid-state energy conversion methods, thermoelectricity deals with direct inter-conversion of thermal and electrical energy. The efficiency of a thermoelectric heat engine is related to a material dependent figure of merit, Z, given by S2σ/κ, where S is the thermopower or Seebeck coefficient, σ and κ are the electrical and thermal conductivities (lattice and electronic), respectively. In conventional thermoelectric materials such as semiconductors, the density of states, chemical potential and the scattering mechanism governs electrical conductivity and thermopower. Due to this coupling between thermopower, electrical conductivity and electronic component of thermal conductivity, achieving high Z has been a challenging task. There are several proposals and/or demonstrations to design high efficiency thermoelectric materials such as “electron crystal, phonon glass” paradigm,  band engineering,  quantum confinement,  electron filtering  etc. The decoupling of lattice component of thermal conductivity from other parameters has been a very successful means to enhance the figure of merit, but further decoupling of parameters like electrical conductivity and thermopower is necessary to achieve high efficiency thermoelectric materials. From the technological perspective, it is essential to design highly stable, cheap and abundant materials with good efficiency for thermoelectric power generation. Complex oxides are an interesting class of materials, which provides a chemically tunable platform to realize a wide range of physical phenomena such as high temperature superconductivity (cuprates), ferroelectricity (titanates, ferrites), magnetism (manganites, cobaltates) etc. Due to this versatility, they can cater to both the scientific questions on thermoelectricity besides providing useful materials for technological applications. The aim of our research during the fellowship was to understand the nature of thermoelectricity in complex oxides and tailor these materials to show enhanced thermoelectric properties using existing or new physical principles discussed earlier. The specific goals of my research was two fold: (1) Understand the cross coupling between thermopower and electrical conductivity in complex oxides and identify the possibilities of decoupling and simultaneously increasing both properties with tunable material parameters. (2) Explore the limits of phonon scattering at the interfaces of the oxide surfaces, particularly in the form of superlattices and use it to enhance the thermoelectric response in their doped analogues.